1. Field of the Invention
Methods consistent with the present invention relate to measuring round-trip-time (RTT) and proximity checking using the same.
2. Description of the Related Art
FIG. 1A is an exemplary diagram illustrating contents transmission. Referring to FIG. 1A, contents are transmitted to a device A from a contents provider CP. The device A is authorized to access the contents. Unlimited distribution of the contents to a device C may not be allowed, even if the contents are transmitted by an authorized user. For example, if the device A functions as a home server of the home network HN, the contents are transmitted within the home network HN. The home network HN includes a device B but not the device C.
Proximity checking is widely used to prevent unlimited distribution of contents.
Proximity checking is performed to determine the proximity between a device (hereinafter referred to as “sink device”) that receives contents (or information whose unlimited distribution is not allowed) and a device (hereinafter referred to as “source device”) which transmits the contents. If both devices are determined to be proximate to each other, contents transmission is allowed; if not, contents transmission is not allowed.
The proximity check is performed using round-trip-time (RTT). The source device measures RTT to the sink device, determines whether the measured RTT is smaller than a critical RTT, and if the measured RTT is determined to be smaller than the critical RTT, determines that the source device and the sink device are proximate to each other. For example, if the critical RTT is 7 ms, the range of the content distribution is restricted to an apartment area.
FIG. 1B is a flowchart illustrating a conventional method of measuring RTT. Referring to FIG. 1B, in Operation 110, a device A generates a first random number R1, and securely transmits the generated first random number R1 to a device B. The term “securely” means that although an external attacker may intercept a message, the first random number R1 cannot be obtained by the external attacker. Such a secure transmission is performed using a public key infrastructure (PKI).
In Operation 120, the device B transmits an acknowledge message OK to the device A.
In Operation 130, the device A generates a second random number R2, transmits the generated second random number R2, and starts a timer for measuring RTT.
In Operation 140, the device B receives the second random number R2 from the device A, generates R1⊕R2, and transmits the generated R1⊕R2 to the device A. The ⊕ means an XOR operation.
The device A receives the R1⊕R2 from the device B, ends the timer, and measures RTT. The device B does not transmit the second random number R2 but R1⊕R2 to the device A in order to prevent an attacker from intercepting the message between the devices A and B, transmitting a new message to the device A or device B, and faking RTT.
However, the conventional method of measuring RTT needs to securely transmit the first random number R1 for one-time RTT measurement every time. That is, the device A encrypts the first random number R1 using a public key of the device B and decrypts the encrypted first random number using its own private key, thereby obtaining the first random number.
The RTT measurement for one-time proximity checking is repeatedly performed several tens of times through several thousands of times. This is because, if one of the measured RTTs is smaller than the critical RTT, after the RTT is measured several tens of times through several thousands of times, the devices A and B are considered to be proximate to each other due to variability of traffic on a transmission path. However, since the conventional method of measuring RTT must perform encryptions and decryptions several tens of times through several thousands of times for the one-time proximity check, it is very inefficient and places considerable load on both systems of the devices A and B.